Imaging device calibration methods, imaging device calibration instruments, imaging devices, and articles of manufacture

ABSTRACT

Imaging device calibration methods, imaging device calibration instruments, imaging devices, and articles of manufacture are described. According to one embodiment, an imaging device calibration method includes emitting light for use in calibration of an imaging device, providing an emission characteristic of the light, sensing the light using an image sensor of the imaging device, generating sensor data indicative of the sensing using the image sensor, and determining at least one optical characteristic of the imaging device using the generated sensor data and the emission characteristic for use in calibration of the imaging device, and wherein the at least one optical characteristic corresponds to the image device used to sense the light.

FIELD OF THE DISCLOSURE

Aspects of the disclosure relate to imaging device calibration methods,imaging device calibration instruments, imaging devices, and articles ofmanufacture.

BACKGROUND OF THE DISCLOSURE

Imaging systems of various designs have been used extensively forgenerating images. Exemplary imaging systems include copiers, scanners,cameras, and more recently digital cameras, and other devices capable ofgenerating images. Color imaging systems have also experiencedsignificant improvements and are increasing in popularity. Color imagingsystems may be calibrated to increase accuracy of various imageprocessing algorithms (e.g., illuminant estimation, color correction,etc.), and also to increase the color accuracy of final reproductions.

For example, even identically configured imaging systems may vary fromone another due to product tolerances or design variances. Referring toFIG. 1, a graphical representation of relative responsivity versuswavelength is shown for two hundred digital cameras corresponding to thesame product. FIG. 1 illustrates the variations in blue, green, and redsensor responsivities of the sampled cameras represented by respectivebands 4, 6 and 8. The illustrated bands have widths illustrating thesize of the variations between respective cameras although the camerasstructurally comprise the same components.

One color calibration technique uses reflective charts. Reflectivecharts can be utilized to calibrate a camera quickly and they arerelatively inexpensive. However, calibrations implemented usingreflective charts may not be accurate enough for utilization withcameras. Monochromators, on the other hand, can produce very accuratecalibrations of color imaging systems including cameras. However, thecalibration procedure with monochromators may take a relatively longperiod of time to complete and the devices are expensive.

At least some aspects of disclosure are related to improved calibrationsystems and methods.

SUMMARY

According to some aspects, exemplary imaging device calibration methods,imaging device calibration instruments, imaging devices, and articles ofmanufacture are described.

According to one embodiment, an imaging device calibration methodincludes emitting light for use in calibration of an imaging device,providing an emission characteristic of the light, sensing the lightusing an image sensor of the imaging device, generating sensor dataindicative of the sensing using the image sensor, and determining atleast one optical characteristic of the imaging device using thegenerated sensor data and the emission characteristic for use incalibration of the imaging device, and wherein the at least one opticalcharacteristic corresponds to the image device used to sense the light.

According to another embodiment, an imaging device calibrationinstrument comprises a light source configured to emit light having aplurality of different spectral power distributions, an opticalinterface configured to provide the light to an imaging device to becalibrated using the imaging device calibration instrument, andprocessing circuitry configured to automatically control the emission oflight from the light source to permit the calibration of the imagingdevice.

Other embodiments are described as is apparent from the followingdiscussion.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of responsivity of a sampling ofimaging systems.

FIG. 2 is an illustrative representation of an exemplary calibrationinstrument and imaging device according to an illustrative embodiment.

FIG. 3 is a functional block diagram of circuitry of a calibrationinstrument according to one embodiment.

FIG. 4 is a functional block diagram of circuitry of an imaging deviceaccording to one embodiment.

FIG. 5 is an illustrative representation of an optical interface of acalibration instrument according to one embodiment.

FIG. 6 is a graphical representation of radiance versus wavelength forlight emitted from the optical interface according to one embodiment.

FIG. 7 is a flow chart representing an exemplary imaging devicecalibration method according to one embodiment.

FIG. 8 a is a flow chart representing exemplary data acquisitionaccording to one embodiment.

FIG. 8 b is a flow chart representing exemplary data acquisitionaccording to another embodiment.

FIG. 9 is a flow chart representing exemplary data processing accordingto one embodiment.

FIG. 10 is a graphical representation comparing exemplary calibrationtechniques.

FIG. 11 is a graphical representation comparing estimated and measuredrelative responsivities using a Macbeth chart calibration technique.

FIG. 12 is a graphical representation comparing estimated and measuredrelative responsivities using a MacbethDC chart calibration technique.

FIG. 13 is a graphical representation comparing estimated and measuredrelative responsivities using an emissive calibration instrumentaccording to one embodiment.

DETAILED DESCRIPTION

At least some aspects of the disclosure provide apparatus and methodswhich enable fast and accurate calibration of an imaging device. In oneembodiment, optical characteristics such as a responsivity functionand/or a transduction function of an imaging device may be measured todetermine how the associated imaging device responds to input lightsignals. The determined optical characteristics may be utilized tocalibrate the respective imaging device. According to exemplaryimplementations, emissive light sources as opposed to reflectivearrangements are used to determine the optical characteristics and whichenable real time fast and relatively inexpensive calibration of animaging device (e.g., on an assembly line).

Referring to FIG. 2, an imaging system 10 according to one embodiment isshown. The depicted imaging system 10 includes an exemplary imagingdevice calibration instrument 12 and an imaging device 14. Instrument 12may be referred to as an emissive calibration instrument in at least oneembodiment wherein one or more light source of the instrument 12 emitslight which is used for implementing determination of calibration dataand calibration of a device 14.

In at least one embodiment, calibration instrument 12 is used to providecalibration data which may be utilized to calibrate imaging device 14.In at least some embodiments described herein, calibration instrument 12may operate in conjunction with imaging device 14 to provide thecalibration data. Calibration data includes optical characteristics suchas responsivity and/or transduction functions of the respective imagingdevice 14 in exemplary embodiments. The calibration data may be utilizedto calibrate the individual respective device 14 used to obtain thecalibration data. For example, image processing algorithms of imagingdevice 14 may be tailored to improve imaging operations thereofincluding the ability of imaging device 14 to produce pleasing andfaithful images of captured scenes.

Imaging device 14 comprises a color digital camera in the illustratedsystem. Other configurations of imaging device 14 configured to generateimage data responsive to received images are possible (e.g., scanner,color copier, color multiple function peripheral, etc.).

Referring again to calibration instrument 12, the depicted exemplaryembodiment includes a light source 20, a light randomizer 22, and anoptical diffuser 24. For ease of discussion, exemplary components 20,22, 24 are shown in exploded view. In typical implementations ofcalibration instrument 12, components 20, 22, 24 are sealed with respectto one another to prevent the introduction of ambient light intoinstrument 12. Processing circuitry of calibration instrument 12 mayalso be provided to control calibration operations as is discussed belowwith respect to the exemplary circuitry of FIG. 3.

Light source 20 may be embodied in different configurations in differentembodiments of calibration instrument 12. Further, light source 20 maybe controlled in different embodiments to emit different lightsimultaneously and/or sequentially. Different light comprises lighthaving different emission characteristics, such as differentwavelengths, intensities or spectral power distributions.

For example, the depicted configuration of light source 20 comprises aplurality of regions 26 which are individually configured to emit lighthaving different wavelengths and/or intensities compared with otherregions 26. Accordingly, the light of at least some of regions 26 may beboth spatially and spectrally separated from light of other regions 26in the embodiment of calibration instrument 12 in FIG. 2. In someembodiments, the light having different wavelengths and/or intensitiesmay be emitted simultaneously. In other embodiments, some of which aredescribed below, light having different wavelengths and/or intensitiesmay be emitted sequentially.

Individual ones of the regions 26 may comprise one or more lightemitting device (not shown). Exemplary light emitting devices includenarrow-band devices which provide increased accuracy compared withbroad-band reflective patches. Light emitting devices of regions 26include light emitting diodes (LEDs) and lasers in exemplaryembodiments. Other configurations of light emitting devices of regions26 may be utilized. In one example, individual regions 26 comprise a 3×3square of light emitting devices configured to emit light of the samewavelength and intensity.

In the depicted exemplary embodiment, light randomizer 22 comprises aplurality of hollow tubes corresponding to respective ones of regions 26of light source 20. Light randomizer 22 is configured to presentsubstantially uniform light for individual ones of regions 26 todiffuser 24 in the described configuration. Internal surfaces of thetubes of light randomizer may have a relatively bright white mattesurface. Other configurations of light randomizer 22 are possible. Forexample, light randomizer 22 may comprise a single hollow tube in atleast one other embodiment of instrument 12 having a single lightemitting region described below.

Optical diffuser 24 comprises an optical interface 27 configured topresent substantially uniform light for individual ones of regions 26(and respective regions 28 of optical interface 27 discussed below) toimaging device 14 for use in calibration operations. Otherconfigurations of optical interface 27 apart from the illustratedoptical diffuser 24 may be utilized to output light to imaging device14. An exemplary optical diffuser 24 comprises a translucent acrylicmember. The illustrated exemplary optical diffuser 24 is configured tooutput light corresponding to light emitted by light source 20. Forexample, the exemplary depicted optical interface 27 comprises aplurality of regions 28 corresponding to respective regions 26 of lightsource 20. In other embodiments, more or less regions 28 may be providedcorresponding to the provided number of regions 26 of light source 20.In at least one embodiment, optical randomizer 22 and diffuser 24provide different light corresponding to respective ones of regions 28and for individual ones of the regions 28, the respective light issubstantially uniform throughout the area of the respective region 28.In other possible implementations, another optical diffuser may beimplemented intermediate light source 20 and light randomizer 22 orwithin light randomizer 22.

In one embodiment, light randomizer 22 comprises plural aluminumsubstantially square tubes corresponding to regions 26 of light source20. The tubes may individually have a length of 2.5 inches betweensource 20 and interface 27 and square dimensions of 1 inch by 1 inch.The interior surfaces of the tubes may be coated with a white coatingsuch as OP.DI.MA material having part number ODMO1-FO1 available fromGigahertz-Optik. Diffuser 24 may comprise a plurality of pieces of whitetranslucent acrylic material having part number 020-4 available fromCyro Industries with dimensions of 1 inch by 1 inch comprisingindividual ones of regions 28 and individually having a thickness of 1/8inch. Other configurations or embodiments are possible.

Referring to FIG. 3, exemplary circuitry 30 of calibration instrument 12is shown. The depicted circuitry 30 includes a communications interface32, processing circuitry 34, storage circuitry 36, light source 20 and alight sensor 38. More, less or alternative circuit components may beprovided in other embodiments.

Communications interface 32 is configured to establish communications ofcalibration instrument 12 with respect to external devices. Exemplaryconfigurations of communications interface 32 include a USB port, serialor parallel connection, IR interface, wireless interface, or any otherarrangement capable of uni or bi-directional communications. Anyappropriate data may be communicated using communications interface 32.For example, as described below, communications interface 32 may beutilized to communicate one or more emission characteristic of lightsource 20 and/or one or more determined optical characteristics of therespective imaging device 14 to be calibrated.

In one embodiment, processing circuitry 34 may comprise circuitryconfigured to implement desired programming. For example, processingcircuitry 34 may be implemented as a processor or other structureconfigured to execute executable instructions including, for example,software and/or firmware instructions. Other exemplary embodiments ofprocessing circuitry include hardware logic, PGA, FPGA, ASIC, statemachines, and/or other structures. These examples of processingcircuitry 34 are for illustration and other configurations are possible.

Processing circuitry 34 may be utilized to control operations ofcalibration instrument 12. In one embodiment, processing circuitry 34 isconfigured to automatically control the timing of emission of light fromthe instrument 12 (e.g., control the timing to simultaneously and/orsequentially emit light having different wavelengths and/or intensitiesfrom instrument 12). In one embodiment, processing circuitry 34 mayautomatically control the timing and the emission of the light withoutuser intervention.

Storage circuitry 36 is configured to store electronic data and/orprogramming such as executable instructions (e.g., software and/orfirmware), calibration data, or other digital information and mayinclude processor-usable media. In addition to the calibration datadescribed above, additional exemplary calibration data may include oneor more emission characteristics of light emitted using opticalinterface 27 of calibration instrument 12. As discussed below, exemplaryemission characteristics include spectral power distributions (SPDs) oflight emitted at optical interface 27 according to one embodiment.Spectral power distributions include emission characteristics includingwavelengths of the emitted light and associated intensities of the lightfor the respective wavelengths of light.

Processor-usable media includes any article of manufacture which cancontain, store, or maintain programming, data and/or digital informationfor use by or in connection with an instruction execution systemincluding processing circuitry in the exemplary embodiment. For example,exemplary processor-usable media may include any one of physical mediasuch as electronic, magnetic, optical, electromagnetic, infrared orsemiconductor media. Some more specific examples of processor-usablemedia include, but are not limited to, a portable magnetic computerdiskette, such as a floppy diskette, zip disk, hard drive, random accessmemory, read only memory, flash memory, cache memory, and/or otherconfigurations capable of storing programming, data, or other digitalinformation.

Light source 20 may be configured in exemplary arrangements as describedabove. For example, light source 20 may be configured to emit light ofdifferent wavelengths and/or intensities in one embodiment. Thedifferent wavelengths and/or intensities may be defined by a pluralityof regions 26 as described above. In another embodiment, light source 20is configured to emit light of a substantially constant wavelengthand/or intensity and a plurality of spatially separated filterspositioned downstream of light source 20 and corresponding to regions 26may be utilized to provide light of any different desired wavelengthsand/or intensities. In another embodiment described below, light source20 may be configured to sequentially emit different light using a singleregion. Other arrangements are possible.

Light sensor 38 is optically coupled with light source 20 and isconfigured to receive emitted light therefrom. In one example, lightsensor 38 is implemented as a photodiode although other configurationsare possible. One or more light sensor 38 may be positioned within lightrandomizer 24 in some embodiments (e.g., one light sensor 38 may bepositioned in light randomizer 22 implemented as a single hollow tube inone exemplary configuration described herein). In other arrangementshaving plural regions 26, light sensor 38 may be optically coupled viaan appropriate light pipe (not shown) or other configuration with theregions 26 and corresponding to emitted light having differentwavelengths and/or intensities.

Light sensor 38 is configured to monitor emitted light for calibrationpurposes of calibration instrument 12 in one arrangement. For example,at least some configurations of light source 20 may provide light whichdrifts in wavelength and/or intensity over time. Light sensor 38 may beutilized to monitor the light and indicate to a user that instrument 12is out of calibration and service is desired. For example, calibrationinstrument 12 may be considered to be out of calibration if intensitiesof different wavelengths of light vary with respect to one another.Exemplary recalibration of calibration instrument 12 may includere-determining the emission characteristics (e.g., spectral powerdistributions) of light emitted from the optical interface 27.

Referring to FIG. 4, imaging device 14 is illustrated in an exemplaryconfiguration as a digital camera. As mentioned previously, imagingdevice 14 may be embodied in other configurations to generate imagesfrom scenes or received light. Imaging device in the illustratedconfiguration includes processing circuitry 40, storage circuitry 42, astrobe 44, an image sensor 46, a filter 48, optics 50, and acommunications interface 52.

In one embodiment, processing circuitry 40 may be embodied similar toprocessing circuitry 34 described above and comprise circuitryconfigured to implement desired programming. Other exemplary embodimentsof processing circuitry include different and/or alternative hardware tocontrol operations of imaging device 14 (e.g., control strobe 44, optics50, data acquisition and storage, processing of image data,communications with external devices, and any other desired operations).These examples of processing circuitry 40 are for illustration and otherconfigurations are possible.

Storage circuitry 42 is configured to store electronic data (e.g., imagedata) and/or programming such as executable instructions (e.g., softwareand/or firmware), or other digital information and may includeprocessor-usable media similar to the above-described storage circuitry36 in at least one embodiment.

Strobe 44 comprises a light source configured to provide light for usagein imaging of operations. Processing circuitry 40 controls operation ofstrobe 44 in the described embodiment. Strobe 44 may be disabled,utilized alone or in conjunction with other external sources of light(not shown).

Image sensor 46 is configured to provide raw image data of a pluralityof raw images. The raw image data comprises digital data correspondingto a plurality of pixels of the raw images formed by image sensor 46.For example, the raw images comprise bytes corresponding to the colorsof red, green and blue at respective pixels in an exemplary RGBapplication. Other embodiments may utilize or provide other colorinformation. Image sensor 46 may comprise a plurality of photosensitiveelements, such as photodiodes, corresponding to the pixels andconfigured to provide the raw digital data usable for generating images.For example, image sensor 46 may comprise a raster of photosensitiveelements (also referred to as pixel elements) arranged in 1600 columnsby 1280 rows in one possible configuration. Other raster configurationsare possible. Photosensitive elements may individually comprise chargecoupled devices (CCDs) or CMOS devices in exemplary configurations. Inone specific example, image sensor 46 may utilize X3 technology insensor arrangements available from Foveon, Inc.

Filter 48 is provided upstream of image sensor 46 to implement anydesired filtering of light received by imaging device 14 prior tosensing by image sensor 46. For example, in one embodiment, filter 48may remove infrared light received by imaging device 14.

Optics 50 includes appropriate lens and an aperture configured to focusand direct received light for creation of images using image sensor 46.Appropriate motors (not shown) may be controlled by processing circuitry40 to implement desired manipulation of optics 50 in one embodiment.

Communications interface 52 is configured to establish communications ofimaging device 14 with respect to external devices (e.g., calibrationinstrument 12). Exemplary configurations of communications interface 52include a USB port, serial or parallel connection, IR interface,wireless interface, or any other arrangement capable of uni orbi-directional communications. Communications interface 52 may beconfigured to couple with and exchange any appropriate data withcommunications interface 32 of calibration instrument 12 or otherexternal device. For example, communications interface 52 may beutilized to receive one or more emission characteristic of light source20 and/or one or more determined optical characteristic of therespective imaging device 14. Further, interface 52 may output sensordata generated by image sensor 46 and which may be used to implementimage processing operations including determination of opticalcharacteristics of imaging device 14 as described below.

Referring to FIG. 5, an exemplary configuration of optical interface 27is shown. The depicted optical interface 27 corresponds to theembodiment of calibration instrument 12 shown in FIG. 2 and includes aplurality of regions 28 of different light having different wavelengthsand/or intensities.

In the illustrated configuration, optical interface 27 includes pluralrows 60 of colored regions and a single row 62 of white regions. More,less or regions of other wavelengths and/or intensities may be providedin other embodiments of optical interface 27.

Colored region rows 60 provide plural regions 28 of light havingdifferent wavelengths. For example, in the depicted embodiment, rows 60include regions 28 sequentially increasing in wavelength at incrementsof 25 nm from ultraviolet light (375 nm) to infrared light (725 nm)providing light which is spectrally and spatially separated. In theillustrated example, row 62 comprises a plurality of regions W1-W5 ofthe same relative spectral power distribution and which increase inintensity. The relative intensity of the white patches may be 0.01,0.03, 0.10, 0.30, and 1 for respective ones of regions W1-W5.

According to the exemplary embodiment of FIG. 5, the number of lightemitting devices and/or the drive currents for the light emittingdevices may be varied between respective regions 28 to provide thedesired spectral power distributions of emitted light. Otherconfigurations are possible in other embodiments.

In one embodiment, the regions 28 of FIG. 5 may be numbered 1 to 15sequentially from left to right for each of the rows starting with thetop row and continuing to the bottom row. Exemplary light emittingdevices may comprise LEDs available from Roither Lasertechnik and havethe following part numbers for the respective regions 28: (1) 380D30,(5) HUBG-5102L, (13) ELD-670-534, (14) ELD-700-534, and (15)ELD-720-534. Remaining exemplary light emitting devices may compriseLEDs available from American Opto and have the following part numbersfor the respective regions 28: (2) L513SUV, (3) L513SBC-430NM, (4)L513NBC, (6) L513NBGC, (7) L513NPGC, (8) L513UGC, (9) L513NYC-E, (10)L513UOC, (11) L513NEC, (12) L513TURC, and (W1-W5) L513NWC.

In this example, the drive currents may be constant for the lightemitting devices of all of the regions 28 for rows 60 (e.g., 18-20 mA)and the number of light emitting devices per region 28 are variedaccording to: (1) 4, (2) 1, (3) 14, (4) 2, (5) 4, (6) 3, (7) 1, (8) 27,(9) 3, (10) 2, (11) 1, (12) 2, (13) 2, (14) 2, and (15) 1. The number oflight emitting devices for individual ones of the regions 28 of row 62may be the same (e.g., four) and the following exemplary drive currentsmay be used: 0.2, 0.6, 2, 6 and 20 mA for respective ones W1-W5 ofregion 28. The above example is for illustration and otherconfigurations or variations are possible.

As described further below, utilization of optical interface 27 shown inFIG. 5 including regions 28 of varying wavelength and/or intensityenables simultaneous determination of responsivity and transductionfunctions of imaging device 14, for example, via a single exposure ofthe device 14 to light emitted from optical interface 27 using imagingdevice 14. Other configurations of optical interface 27 are possible asdiscussed herein (e.g., providing an optical interface wherein onlywavelength or intensity are varied between regions 26, providing anoptical interface with only a single emission region for sequentiallyemitting light of the same wavelength and/or intensity, etc.).

Provision of light of different wavelengths by calibration instrument 12may be utilized to determine a responsivity function of imaging device14. In the embodiment of optical interface 27 illustrated in FIG. 5,plural regions 26 of rows 60 may simultaneously emit light fordetermination of the responsivity function via a single exposure theretoby imaging device 14 due to the spatially and spectrally separatedregions 26 of rows 60.

Referring to FIG. 6, the emission of light via optical interface 27(i.e., and received by imaging device 14) may be optimized to facilitatedetermination of the responsivity function of the imaging device 14being calibrated. The graphical representation of FIG. 6 illustratesspectral power distributions of light emitted by light source 20 andprovided at regions 28 of optical interface 27 which facilitate theresponsivity analysis of imaging device 14. The spectral powerdistributions include exemplary radiance values for the regions 28 ofoptical interface 27 depicted in FIG. 5 increasing in wavelength fromleft to right along the x-axis.

As mentioned above, the number of light emitting devices of source 20may be varied for individual regions 26 to provide differentintensities. In another embodiment, the number of light emitting devicesmay be the same for individual regions 26 and the drive currents of thelight emitting devices of the respective regions 26 may be varied toprovide desired intensities. Other arrangements may be used to providedesired spectral power distributions. In one embodiment, the intensitiesmay be selected to approximate the exemplary spectral powerdistributions depicted in FIG. 6 during calibration of instrument 12itself. Once the appropriate drive currents of the light emittingdevices of respective regions 26 (or other configuration parameters) aredetermined, instrument 12 may be calibrated to drive the light emittingdevices using the determined drive currents or parameters. In oneembodiment, the light emitting devices of a respective region 26 may bedriven using the same drive current while drive currents used to drivelight emission devices of different regions 26 may be different. Otherconfigurations apart from varying the number of light emitting devicesand/or drive currents for respective regions 26 may be used in otherembodiments as mentioned above.

Further, the spectral power distribution of light emitted at opticalinterface 27 using the drive currents may be determined followingcalibration of instrument 12. In one example, the spectral powerdistribution of light emitted at optical interface 27 may be measuredusing a spectral radiometer. The measured spectral power distribution ofcalibration instrument 12 may be stored as an emission characteristic ofcalibration instrument 12 using storage circuitry 36 or otherappropriate circuitry and subsequently utilized during calibrationoperations of one or more imaging device 14. New drive currents and/orspectral power distributions may be determined during recalibration ofinstrument 12.

Emission characteristics may also be provided and stored for individualregions 28 of row 62. As mentioned previously, at least some of theregions 28 may be configured to vary intensity of light for a givenwavelength of light (e.g., the regions of row 62). Data regarding theintensities of light corresponding to regions 28 may be stored as anemission characteristic for subsequent usage in calibration of one ormore imaging device 14. The intensity data may also be extracted fromthe spectral power distributions of light from regions 28 within row 62.

Referring to FIG. 7, an exemplary method for implementing calibration ofan imaging device 14 using calibration instrument 12 is shown. Othermethods are possible including more, less or alternative steps.

At a step S1, an embodiment of calibration instrument 12 having a lightsource is provided along with at least one emission characteristic oflight emitted from the light source.

At a step S2, the imaging device 14 to be calibrated is aligned withcalibration instrument 12.

At a step S3, image sensor 46 of imaging device 14 is exposed to lightemitted from the light source.

At a step S4, image sensor 46 senses the light and generates sensor datawhich is indicative of the sensing by the image sensor 46.

At a step S5, appropriate processing circuitry determines an opticalcharacteristic of imaging device 14 using the emission characteristicand the sensor data. The optical characteristic may be utilized tocalibrate imaging device 14. The exemplary method of FIG. 7 may berepeated for other imaging devices 14.

Referring to FIG. 8 a, a flow chart illustrates an exemplary method fordata acquisition during calibration of an associated imaging device 14using the calibration instrument 12 described with reference to FIG. 2.

At a step S10, the imaging device to be calibrated is brought intoalignment to receive light emitted from the optical interface of thecalibration instrument 12. Once aligned, the light source 20 ofcalibration instrument 12 is controlled to emit light at regions 28 ofoptical interface 27. Imaging device 14 is configured to provide theoptical interface 27 into focus and to expose the image sensor 46 tolight from calibration instrument 12 (e.g., takes a photograph) toreceive the light emitted from optical interface 27.

At a step S12, sensor data is generated by image sensor 46 responsive tothe exposing in step S10. In one embodiment, individual pixels of imagesensor 46 are configured to provide sensor data comprising RGB values.Pixel locations of image sensor 46 may correspond to regions 28 ofoptical interface 27. Accordingly, a plurality of pixels of image sensor46 may be identified which correspond to individual ones of regions 28.RGB values from individual ones of the pixels which correspond torespective individual regions 28 and may be averaged using processingcircuitry 34, 40 or other desired circuitry in one embodiment to providea single averaged RGB value for each of regions 28. According to oneembodiment, the sensor data comprising averaged RGB values may beutilized for calibration of imaging device 14 as described below.

Data acquisition operations are described below with respect to anotherembodiment of calibration instrument 12. Calibration instrument 12according to the other presently described embodiment includes anoptical interface having a single region (not shown) to output light forcalibration of imaging device 14. For example, as opposed to arranginglight emitting devices of different wavelengths and/or intensitiesaccording to regions 26 as described above, light emitting devices ofthe light source having different wavelengths or intensities may bedistributed around an entirety of the area of the region of the opticalinterface.

In one embodiment, it is desired for the light emitting devices of thelight source to provide a substantially uniform distribution of lightacross an entirety of the area of the region of the optical interface.In one possible implementation, individual ones of the light emittingdevices comprising twenty different wavelengths or intensities may bepositioned adjacent to one another in sequence in both rows and columnsto provide a substantially uniform emission of light across the regionof the optical interface for individual ones of the wavelengths onintensities. Other patterns of distribution of the light emittingdevices are possible.

In one operational embodiment, only the light emitting devices of acommon wavelength or intensity may be controlled to emit light at anygiven moment in time. According to this embodiment, the light emittingdevices of a first wavelength of light may be controlled to emitrespective light substantially uniform across the area of the region.Thereafter, the light emitting devices for the remaining wavelengths maybe sequentially individually controlled to emit light of the respectivewavelengths in sequence providing temporal and spectral separation ofthe emitted light. If present, light emitting devices having differentintensities for a given wavelength may thereafter be individuallyconfigured to emit light in sequence to enable transduction calibrationoperations described further below. Accordingly, in one embodiment, thelight emitting devices of respective wavelengths or intensities may besequentially configured to emit respective light. More specifically,light emitting devices having a common wavelength may be sequentiallycontrolled to individually emit light starting at 375 nm and progressingto 725 nm and followed by the emission of light from light emittingdevices configured to provide light of a common wavelength and variedintensity from W1 to W5. Imaging device 14 may sense emitted light foreach of the respective emitted wavelengths 375 nm-725 nm and intensitiesW1-W5 of light in one embodiment. Sensor data is then provided byimaging device 14 for each of the wavelengths and intensities of light.

Referring to FIG. 8 b, exemplary data acquisition operations accordingto the second above-described embodiment having an optical interface 27with a single region providing sequentially emitted different light aredescribed.

At a step S20, the calibration instrument is controlled to emit lighthaving a single wavelength. The image sensor of the imaging device to becalibrated is exposed to the emitted light.

At a step S22, an average RGB value for the respective wavelength may bedetermined from pixel sensor data of the image sensor using processingcircuitry 34, 40 or other desired circuitry.

Thereafter, the processing may return to step S20 whereupon theinstrument controls the emission of light of the next wavelengthenabling generation of sensor data for the respective wavelength usingthe imaging device 14. The process of FIG. 8 b may be repeated toprovide sensor data comprising averaged RGB values in the describedembodiment for as many different wavelengths or intensities of lightemitted using the calibration instrument.

The above-described embodiments are provided to illustrate exemplarydata acquisition techniques for implementing imaging device calibrationoperations. Other data acquisition methods and/or apparatus may be usedin the other embodiments.

Referring to FIG. 9, the acquired data is processed followingacquisition to determine calibration data of the imaging device 14.Exemplary processing includes determining calibration data comprisingoptical characteristics (e.g., responsivity and/or transductionfunctions) for the respective imaging device 14 according to oneembodiment. As mentioned above, processing circuitry 34, 40 and/or otherappropriate processing circuitry may perform data acquisitionoperations. Similarly, processing circuitry 34, 40 and/or otherappropriate processing circuitry may be utilized to process the acquireddata for example as shown in FIG. 9. Further, data acquisition andprocessing may be performed by the same or different processingcircuitry.

In the illustrated exemplary processing of FIG. 9, opticalcharacteristics including responsivity and transduction functions of theimaging device 14 are determined. In other embodiments, only one ofresponsivity or transduction functions, and/or alternativecharacteristics of the imaging device 14 are determined. Further,additional optical characteristics or other information for use incalibration of imaging device 14 may be determined. For example,responsivity and/or transduction functions may be further processed byappropriate processing circuitry 34, 40 or other processing circuitry(not shown). For example, a color correction matrix, an illuminantestimation matrix and/or other information may be derived from theresponsivity and transduction functions.

Steps S30-S34 illustrate exemplary processing for determining aresponsivity function of imaging device 14.

Steps S40-S44 illustrate exemplary processing for determining atransduction function of imaging device 14. Other processing may beutilized according to other arrangements (not shown).

At step S30, the sensor data obtained from image sensor 46 including theaveraged RGB values described above for the respective individualregions 28 of rows 60 in the described embodiment may define a matrix r.

At step S32, the emission characteristic comprising spectral powerdistributions (SPDs) of the regions 28 in the described embodiment maydefine a matrix S.

At step S34, the responsivity function R may be determined usingmatrices r, S and the equation R=pinv(S^(T))r^(T) in the describedexample.

The transduction function may be determined in parallel with thedetermination of the responsivity function in the illustrated example.

Referring to step S40, the sensor data from image sensor 46 includingthe averaged RGB values for the respective individual regions 28 of row62 in the described embodiment may define a matrix r_(w).

At step S42, the emission characteristic comprising spectral powerdistributions of the regions 28 in the described embodiment may define amatrix S_(w).

At step S44, the transduction function g(x)−>g(1^(T)S_(w))=r_(w) may besolved using matrices r_(w), S_(w) in the described example.

The above-described methods of FIG. 9 may be used to determine one ormore optical characteristic for respective individual ones of theimaging devices 14 which provided the respective sensor data indicativeof the circuitry of the respective imaging devices 14, and accordingly,the above-described processes may be performed for individual ones ofimaging devices 14 to be calibrated to determine the respectiveappropriate one or more optical characteristic for the respectivedevices 14. The above-described methods of FIG. 9 are exemplary andother processing or methods may be utilized to determine responsivityand/or transduction functions or other optical characteristics ofimaging device 14 in other embodiments.

Once determined, the optical characteristics may be used to calibratethe respective imaging devices 14. For example, optical characteristicscomprising responsivity and transductance functions may be used toincrease the accuracy of image processing algorithms (e.g., illuminantestimation and color correction) of respective imaging devices 14, andalso to increase the color accuracy of final reproductions.

As described herein in one embodiment, the exemplary apparatus and/ormethods may be used to determine whether components of imaging device 14are defective (e.g., sensor 46, filter 48, etc.). For example, theability of the respective imaging devices 14 to remove infrared or otherlight may also be monitored using calibration instruments 12 discussedabove and configured to emit infrared or other light. For example, afilter of imaging device 14 and configured to remove certain light(e.g., infrared) may be identified as defective if the sensor datagenerated by the respective imaging device 14 responsive to lightemitted from optical interface 27 of calibration instrument 12 (andincluding infrared or other desired light) indicates that the receivedlight included emitted infrared or the other light which was not removedby filter 48.

In one embodiment, the determined optical characteristics may becommunicated to respective imaging devices 14 which implementappropriate calibration if the optical characteristics were determinedusing processing circuitry 34 of calibration instrument 12 (or otherprocessing circuitry external of imaging devices 14). Alternately,processing circuitry 40 of imaging devices 14 may determine the opticalcharacteristics of the respective devices 14. In another embodiment, thecalibration may be performed externally of imaging devices 14 using thedetermined optical characteristics and the calibrated image processingalgorithms may be subsequently provided to the respective imagingdevices 14. In yet another embodiment, processing circuitry 40 ofimaging devices 14 may be configured to utilize the determined (e.g.,internally or externally) optical characteristics to implement thecalibration internally of the imaging devices 14. In sum, anyappropriate processing circuitry may be configured to generate one ormore optical characteristic for the respective imaging devices 14 andthe same or other processing circuitry may utilize the one or moreoptical characteristic to implement the calibration.

Referring to FIG. 10, a graphical representation is shown of singularvalue decomposition of different calibration methods including exemplaryemissive aspects described herein compared with usage of reflectivepatches (Macbeth and Macbeth DC) and a monochromator.

The relatively high and constant singular value decomposition using theexemplary emissive calibration instrument 12 of FIG. 2 and describedherein is similar to results achieved with a monochromator and greatlyexceed the results achieved through the Macbeth and Macbeth DCreflective patches wherein the respective curves are not constant andhave relatively rapidly decreasing slopes. The accuracy of thecalibration methods depends on how spectrally correlated the reflectivepatches or the light emitting devices are to each other. More correlatedpatches or light emitting devices produce less accurate calibrations.This is the case because calibration techniques invert an imageformation equation to compute the camera responsivity functions. Whenspectrally correlated patches or light emitting devices are inverted,noisy estimates of the camera responsivity functions result. Thesingular values of the reflectance functions of patches or the spectralpower distributions of light emitting devices indicate the accuracy of agiven method. The more singular values which are greater than 0.01(anything less may be considered too noisy), the more accurate themethod (see e.g., FIG. 10). Basically, the number of singular valuesindicates the number of patch colors or light emitting devices thatcontribute to the resulting calibration.

Further, with respect to FIGS. 11-13, exemplary relative responsivitiesdetermined using Macbeth reflective patches (FIG. 11), MacbethDCreflective patches (FIG. 12) and the exemplary emissive calibrationinstrument 12 of FIG. 2 (FIG. 13) for a D1 digital camera available fromNikon are individually shown with respect to graphs measured using amonochromator. It is clear from a comparison of FIGS. 11-13 that thecalibration instrument 12 of FIG. 2 provides increased accuracy ofdetermining relative responsivities of a given imaging device 14compared with usage of reflective patches (e.g., Macbeth and MacbethDC).

Table 1 compares the calibration procedures using reflective charts, thecalibration instrument 12 of FIG. 2 and a monochromator. The calibrationinstrument 12 of FIG. 2 provides the shortest calibration time for agiven imaging device 14 (i.e., slightly shorter than the reflectivechart) and no uniformity of an external light source is required as withthe reflective chart, and hours shorter than a monochromator (i.e.,colors may be measured spatially in the configuration of FIG. 2 insteadof temporally as with the monochromator). Calibration instrument 12 hasthe shortest calibration time of the compared devices since externalsources of light do not have to be made uniform (e.g., the exemplaryinstrument 12 emits desired light itself). TABLE 1 CalibrationReflective chart Instrument Monochromator 1. Uniformly illuminate 1.Turn on the 1. Set monochromator to a    the chart using    device.   specified wavelength and    an ambient source. 2. Take a    bandwidth.2. Take a photograph    photograph of 2. Take a photograph of the    ofthe chart    the device.    light exiting the 3. Run software 3. Run   monochromator.    to calibrate.    software 3. Measure the power level   to calibrate    of the light exiting the    monochromator. 4. Repeatsteps 1-3 for    each wavelength of    the visible spectrum. 5. Runsoftware to    calibrate.

Table 2 compares approximate cost of devices configured to implement theabove-described three calibration methods. TABLE 2 Reflective chartCalibration Instrument Monochromator $50-$350 (retail) $200-$400 (est.retail) $5,000-$20,000 (retail)

Table 3 compares the number of singular values of the three methods anddevices including the calibration instrument of FIG. 12. Otherembodiments of calibration instrument 12 may include more or lesswavelengths and/or intensities of light as desired. For example,embodiments of instrument 12 described above include twenty types ofdifferent light. In other embodiments, any appropriate number ofdifferent types of light (wavelength and/or intensity) may be usedsequentially, in plural regions, or according to other appropriateschemes. TABLE 3 Reflective chart Calibration Instrument Monochromatorapproximately 4 15-20 (depends on >50 number of emissive sources)

Reflective charts because they have broadband, highly-correlated patchcolors, only contribute approximately 4 measurements that can be usedfor calibration. This is typically not adequate for calibrations ofimaging devices 14 comprising cameras. The monochromator, on the otherhand, produces over 50 calibration measurements because it typicallyuses narrow-band sources. Hence, the monochromator produces calibrationresults of increased accuracy, but the calibration time is relativelylong and the cost is relatively expensive. The exemplary calibrationinstrument 12 of FIG. 2 has an associated 15-20 measurements, forexample, which produces more than adequate calibration results fortypical imaging devices 14 (e.g., digital cameras), but it does notsuffer the cost and long calibration times of the monochromator orutilize external illumination as used with reflective patches.

Accordingly, at least some aspects of the disclosure allow for quick,accurate, and relatively inexpensive determination and calibrations ofresponsivity and transduction functions of imaging devices 14 and may beutilized to calibrate imaging devices on the manufacturing line in atleast one implementation. As discussed above, imaging devices 14 of thesame model or using the same type of components may have differentresponsivity and transduction functions due to sensor and/or colorfilter manufacturing variations. Calibration instruments 12 describedherein may be used for determining optical characteristics of thedevices 14 and calibrating the devices 14 before the imaging devices 14are shipped to a customer or dealer. The relatively quick and accuratecalibrations may improve the overall color reproduction quality ofindividually calibrated imaging devices 14.

Calibration instruments 12 or methods discussed herein may also be usedby professional or prosumer photographers for calibration of high-endimaging devices 14. It is believed that such calibrations would improvethe overall color reproduction quality of the resulting images generatedby such calibrated imaging devices 14. At least some such calibrationaspects may be focused to a more professional market inasmuch as somecalibration aspects utilize raw image data from the imaging device 14and typically, raw image data is provided by imaging devices 14developed for these markets.

The protection sought is not to be limited to the disclosed embodiments,which are given by way of example only, but instead is to be limitedonly by the scope of the appended claims.

1. An imaging device calibration method comprising: emitting light foruse in calibration of an imaging device; providing an emissioncharacteristic of the light; sensing the light using an image sensor ofthe imaging device; generating sensor data indicative of the sensingusing the image sensor; and determining at least one opticalcharacteristic of the imaging device using the generated sensor data andthe emission characteristic for use in calibration of the imagingdevice, and wherein the at least one optical characteristic correspondsto the image device used to sense the light.
 2. The method of claim 1wherein the at least one emission characteristic comprises a spectralpower distribution of the light.
 3. The method of claim 1 wherein the atleast one emission characteristic comprises intensity data.
 4. Themethod of claim 1 wherein the determining comprises determining the atleast one optical characteristic comprising responsivity.
 5. The methodof claim 1 wherein the determining comprises determining the at leastone optical characteristic comprising transduction.
 6. The method ofclaim 1 wherein the determining comprises determining the at least oneoptical characteristic comprising information derived from at least oneof a responsivity function and a transduction function.
 7. The method ofclaim 1 wherein the emitted light comprises infrared light, and furthercomprising sensing for the presence of infrared light.
 8. The method ofclaim 1 further comprising detecting a defective component of theimaging device responsive to the sensing.
 9. The method of claim 1wherein the emitting the light comprises simultaneously emittingdifferent light using a plurality of light emitting devices, and thedetermining comprises determining the at least one opticalcharacteristic comprising responsivity and transduction using thesimultaneously emitted light.
 10. The method of claim 1 wherein theemitting comprises emitting light of different wavelengths.
 11. Themethod of claim 1 wherein the emitting comprises emitting light ofdifferent intensities.
 12. The method of claim 1 wherein the emittingcomprises sequentially emitting different light.
 13. The method of claim1 wherein the emitting comprises simultaneously emitting differentlight.
 14. The method of claim 1 wherein the determining is external ofthe imaging device, and further comprising communicating the at leastone optical characteristic to the imaging device.
 15. The method ofclaim 1 wherein the determining comprises determining using the imagingdevice.
 16. An imaging device calibration method comprising: providingan imaging device; receiving light using the imaging device; generatingsensor data indicative of the receiving of the light using the imagingdevice; providing at least one emission characteristic of the light; andusing the imaging device, determining at least one opticalcharacteristic of the imaging device using the sensor data and the atleast one emission characteristic for use in calibration of the imagingdevice.
 17. The method of claim 16 wherein the at least one emissioncharacteristic comprises a spectral power distribution of the light. 18.The method of claim 16 wherein the at least one emission characteristiccomprises intensity data of the light.
 19. The method of claim 16wherein the determining comprises determining the at least one opticalcharacteristic comprising responsivity.
 20. The method of claim 16wherein the calculating comprises determining the at least one opticalcharacteristic comprising transduction.
 21. The method of claim 16wherein the determining is external of the imaging device, and furthercomprising communicating the at least one optical characteristic to theimaging device.
 22. The method of claim 16 wherein the determiningcomprises determining using the imaging device.
 23. An imaging devicecalibration instrument comprising: a light source configured to emitlight having a plurality of different spectral power distributions; anoptical interface configured to provide the light to an imaging deviceto be calibrated using the imaging device calibration instrument; andprocessing circuitry configured to automatically control the emission oflight from the light source to permit the calibration of the imagingdevice.
 24. The instrument of claim 23 wherein the processing circuitryis configured to automatically control the emission without userintervention.
 25. The instrument of claim 23 wherein the processingcircuitry is configured to automatically control a timing of theemission of light from the light source.
 26. The instrument of claim 23further comprising a communications interface configured to communicatedata regarding at least one emission characteristic of the emitted lightto permit calibration of the imaging device.
 27. The instrument of claim26 wherein the at least one emission characteristic comprises thespectral power distributions.
 28. The instrument of claim 26 wherein theat least one emission characteristic comprises intensity data.
 29. Theinstrument of claim 26 wherein the communications interface isconfigured to receive sensor data generated by the imaging device andwhich is indicative of the emitted light received using the imagingdevice, and the processing circuitry is configured to generate at leastone optical characteristic of the imaging device using the sensor data,and wherein the communications interface is further configured tocommunicate data regarding the at least one optical characteristic ofthe imaging device to the imaging device.
 30. The instrument of claim 29wherein the at least one optical characteristic comprises responsivity.31. The instrument of claim 29 wherein the at least one opticalcharacteristic comprises transduction.
 32. The instrument of claim 23wherein the optical interface comprises a plurality regions comprisingdifferent wavelengths of light.
 33. The instrument of claim 32 whereinthe optical interface comprises a plurality of regions comprisingdifferent intensities of the same wavelength of light.
 34. Theinstrument of claim 23 wherein the optical interface comprises aplurality of regions comprising different intensities of the samewavelength of light.
 35. The instrument of claim 23 wherein the opticalinterface is configured to simultaneously emit the light of thedifferent spectral power distributions.
 36. The instrument of claim 23wherein the optical interface is configured to sequentially emit thelight of the different spectral power distributions.
 37. The instrumentof claim 23 further comprising a light sensor configured to provide dataregarding the different spectral power distributions of the emittedlight to verify calibration of the imaging device calibrationinstrument.
 38. The instrument of claim 23 wherein the optical interfaceis configured to simultaneously emit the light of the different spectralpower distributions to permit simultaneous determination of responsivityand transduction of the imaging device.
 39. The instrument of claim 23wherein the light source is configured to emit light comprising at leastone of infrared, ultra-violet and visible light.
 40. The instrument ofclaim 23 wherein the light source comprises a plurality of lightemitting devices configured to emit the light having the differentspectral power distributions, and the processing circuitry is configuredto control the emission of light from the light emitting devices usingdifferent drive currents.
 41. An imaging device comprising: image meansfor generating sensor data responsive to received light and indicativeof the received light; and processing means coupled with the image meansand comprising means for accessing at least one emission characteristicof the received light and the sensor data generated responsive to thereceived light, and wherein the processing means further comprises meansfor determining at least one optical characteristic of the imagingdevice using the generated sensor data and the at least one emissioncharacteristic.
 42. The device of claim 41 wherein the at least oneemission characteristic comprises spectral power distribution dataincluding intensity data of the received light and the at least oneoptical characteristic comprises responsivity and transduction of theimaging device.
 43. The device of claim 41 wherein the processing meanscomprises means for determining the at least one optical characteristicfor use in calibrating the imaging device.
 44. An article of manufacturecomprising: media configured to store executable instructions configuredto cause processing circuitry to: access sensor data indicative of lightreceived by an imaging device; access at least one emissioncharacteristic of the light received by the imaging device; anddetermine at least one optical characteristic of the imaging deviceusing the accessed sensor data and the accessed at least one emissioncharacteristic.
 45. The article of claim 44 wherein the at least oneemission characteristic comprises spectral power distribution dataincluding intensity data of the light and the at least one opticalcharacteristic comprises responsivity and transduction of the imagingdevice.
 46. The article of claim 44 wherein the processing circuitrycomprises circuitry of the imaging device.
 47. The article of claim 44wherein the processing circuitry comprises circuitry of a calibrationinstrument configured to calibrate the imaging device.